• Greenhouse gas (GHG) emissions

 

The five main sources of direct GHGs emissions from smelters are:

  • Anode combustion during electrolysis (over 80% of GHGs). Oxygen released by electrolytic reaction then reacts with the carbon in the anode to form CO2.
  • Pitch combustion during anode baking.
  • Fuel consumption, such as natural gas, used mostly in the casthouse and for anode baking.
  • Fuel consumption by vehicles.
  • Anode effects during electrolysis.

 

During anode effects, a reaction takes place between the carbon in the anode and fluorides in the bath, leading to emissions of CF4 and C2F6, two gases with a much higher climate-change potential than CO2. This is why GHG reduction measures are principally focused on eliminating anode effects.

 

When calculating GHG inventory, we consider that all of the carbon in the anode will react to form CO2 since the carbon monoxide which results from anode combustion eventually transforms into CO2.

 

Indirect GHG emissions come from electricity consumption at our plants. Indirect GHG emissions from the electricity used at our Québec smelters account for less than 1% of total emissions, since these are powered mainly by hydroelectricity.

 

 


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The closure of the Söderberg potlines at the Baie-Comeau Smelter–a technology dating back to the 1950s–was a key contributor to the reduction of Alcoa’s GHG emissions in Canada from 2013 to 2014 (13%). Emission rates have been maintained in 2015.


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Average process efficiency also improved, decreasing direct emission intensity by 8% from 2013 to 2014. The closure of the Söderberg potlines and the good performance of prebake smelters in controlling anode effects, notably in Baie-Comeau, drove this reduction (see perfluorocarbon emissions graph).


 

 

 

Smelter performance in controlling anode effects, which result in perfluorocarbon (PFC) emissions, is closely monitored. These GHG emissions are manageable through improvements in control systems and alumina feed to the pots. An improvement and exceptional performance was observed at the Baie-Comeau Smelter in 2015. PFC emissions from Alcoa’s Québec smelters are among the lowest in the industry.